Firefly halo on glossy pass?
Here is how to composite your scene. This may also resolve your other question: Posterized shadow pass in Cycles?

Since you only use Glossy and Diffuse shaders, we only need the glossy and diffuse passes enabled in the render passes.



What you are experiencing are not fireflies. These artefacts will not be visible after the correct compositing.

The reason for that is the premultiplication of very transparent pixels after anti-aliasing.

In some edge pixels the popcorn overlaps the box. In a pixel which the box covers just slightly and which is mainly filled by a popcorn the glossy color of the box will be close to 0. The Glossy is going to be multiplied by the Glossy Color. Hence it needs to be unpremultiplied. There are plenty of tutorials on premultiplication and unpremultiplication in compositing and why they are important. Here is an explanation of the technical background. Color correction also only works with unpremultiplied data. Your scene is fine.